1. Field of the Invention
The present invention relates generally to implantable transducers and more particularly relates to implantable transducers having a resistive bridge physically remote from the sensing electronics.
2. Description of the Prior Art
The earliest and, indeed, the currently most prevalent type of pressure measurement systems to be used within the human body use a pressure transmitting catheter which transmits the pressure to be measured between a remote site located within the body and the pressure transducer itself which is located external to the body. Such an apparatus is described in U.S. Pat. No. 3,473,386 issued to Nielsen Jr., et al. The primary disadvantage of this technique is the lack of accuracy achieved by the pressure transmitting catheter. A second disadvantage now becoming prominent is that devices of this type are not implantable and are therefore not suitable for chronic applications.
An early disclosure of an application for a chronically implantable pressure transducer is seen in U.S. Pat. No. 3,614,954 issued to Mirowski et al. The pressure transducer envisioned by Mirowski et al is a piezoresistive bridge circuit. Referring to FIG. 3 of Mirowski et al, one can see the pressure transducer element diagrammed as reference number 40. It is observed that the pressure transducer is a standard four-leg resistive bridge having two connections for supplying power noted -IV and +IV and having two signal connections which are connected to amplifier circuit 42. Though this is a relatively early technique, it is still the most popular technique for connecting an implantable pressure transducer to the associated electronics.
A much later transducer is disclosed in U.S. Pat. No. 4,023,562 issued to Hynecek et al. This reference describes, in considerable detail, the structure of an implantable piezoresistive silicon-based pressure transducer. However, notice at FIG. 1 the four connections that are required. These are brought out to the four metallized conductor pads 9. The specification at column 3, lines 48-50 states, "external readout equipment (not shown) is connected to the external transducer contacts 9 by the wires 12 (FIG. 2)". Hynecek et al. definitely envisioned four separate conductors.
U.S. patent application Ser. No. 186,373 entitled Integral Hermetic Implantable Pressure Transducer by Kenneth M. Anderson filed Sept. 12, 1980 assigned to the assignee of the present patent application discloses a practical, chronically implantable pressure transducer with a hermetically sealed package. Again, the pressure transducer taught by Anderson uses a four-wire system. Two of the wires are used to supply the power to the resistive bridge and the remaining two conductors are used for sensing.
It is desirable to minimize the number of conductors required because this decreases the diameter of the lead required for implantation and increases the reliability over time.